Method for controlling polymerization rate of cyclic polycarbonate oligomers with pretreated catalyst

ABSTRACT

Cyclic polycarbonate oligomers are polymerized to linear polycarbonates by contact with a polycarbonate formation catalyst which has been modified by contacting the same with at least one diaryl carbonate, preferably diphenyl carbonate. Said modification decreases the polymerization rate or introduces an induction period into the polymerization reaction.

This invention relates to the formation of linear polycarbonates fromcyclic polycarbonate oligomers, and more particularly to a method formodifying the rate of the polymerization reaction leading to suchformation.

The conversion of low molecular weight cyclic aromatic carbonatepolymers to linear polycarbonates is known. Reference is made, forexample, to the following U.S. Pat. Nos.:

3,155,683; 3,386,954;

3,274,214; 3,422,119.

More recently, cyclic polycarbonate oligomer mixtures and similarmixtures involving thiol analogs of the carbonates have been preparedand converted to linear polycarbonates, often of very high molecularweight, by contact with various polycarbonate formation catalysts.Reference is made to copending, commonly owned applications Ser. No.704,122, filed Feb. 22, 1985, and Ser. No. 723,672, filed Apr. 16, 1985,now U.S. Pat. No. 4,605,731, the disclosures of which are incorporatedby reference herein. The polycarbonate formation catalysts disclosed asuseful in said applications include various bases and Lewis acids.

The conversion of cyclic polycarbonate oligomer mixtures to linearpolycarbonates has high potential for utilization in reactive processingmethods, such as polymerization in a mold for direct production ofmolded articles. One reason for this is the low melt viscosity of theoligomer mixtures, as a result of which handling thereof is simple andconvenient.

In some polymerization processes, it is desirable to lower the rate ofpolymerization or to introduce an induction period for ease of handling.This is a desirable option for the polymerization of cyclicpolycarbonate oligomer mixtures, a reaction which may take place in fiveminutes or less under normal conditions. If the polymerization ratecould be decreased when desired, such operations as the molding of largeparts could be facilitated.

A principal object of the present invention, therefore, is to provide amethod for controlling the polymerization rate of cyclic polycarbonateoligomers to linear polycarbonates.

A further object is to provide a means to lower the polymerization rateor introduce an induction period.

A further object is to provide a method for modifying the polycarbonateformation reaction so as to facilitate its use under a wide variety ofmolding conditions.

A still further object is to modify polycarbonate formation catalysts soas to accomplish the foregoing.

Other objects will in part be obvious and will in part appearhereinafter.

In one of its aspects, the present invention is a method for modifying apolycarbonate formation catalyst to control the polymerization rate to alinear polymer by contact therewith of at least one cyclic oligomercomprising structural units having formula I in the drawings, whereineach R¹ is independently a divalent aliphatic, alicyclic or aromaticradical and each Y¹ is independently oxygen or sulfur, which comprisesinitially contacting said catalyst with at least one diaryl carbonate ata temperature up to about 350° C.

As will be apparent from the above, the cyclic oligomers useful in thisinvention may contain organic carbonate, thiolcarbonate and/ordithiolcarbonate units. The various R¹ values therein may be differentbut are usually the same, and may be aliphatic, alicyclic, aromatic ormixed; those which are aliphatic or alicyclic generally contain up toabout 8 carbon atoms. Suitable R¹ values include ethylene, propylene,trimethylene, tetramethylene, hexamethylene, dodecamethylene,1,4-(2-butenylene), 1,10-(2-ethyldecylene), 1,3-cyclopentylene,1,3-cyclohexylene, 1,4-cyclohexylene, m-phenylene, p-phenylene,4,4'-biphenylene, 2,2-bis(4-phenylene)propane, benzene-1,4-dimethylene(which is a vinylog of the ethylene radical and has similar properties)and similar radicals such as those which correspond to the dihydroxycompounds disclosed by name or formula (generic or specific) in U.S.Pat. No. 4,217,438, the disclosure of which is incorporated by referenceherein. Also included are radicals containing non-hydrocarbon moieties.These may be substituents such as chloro, nitro, alkoxy and the like,and also linking radicals such as thio, sulfoxy, sulfone, ester, amide,ether and carbonyl. Most often, however, all R¹ radicals are hydrocarbonradicals.

Preferably at least about 60% and more preferably at least about 80% ofthe total number of R¹ values in the cyclic oligomers, and mostdesirably all of said R¹ values, are aromatic. The aromatic R¹ radicalspreferably have formula II, wherein each of A¹ and A² is a single-ringdivalent aromatic radical and Y² is a bridging radical in which one ortwo atoms separate A¹ from A². The free valence bonds in formula II areusually in the meta or para positions of A¹ and A² in relation to Y².Such R¹ values may be considered as being derived from bisphenols of theformula HO--A¹ --Y² --A² --OH. Frequent reference to bisphenols will bemade hereinafter, but it should be understood that R¹ values derivedfrom suitable compounds other than bisphenols may be employed asappropriate.

In formula II, the A¹ and A² values may be unsubstituted phenylene orsubstituted derivatives thereof, illustrative substituents (one or more)being alkyl, alkenyl (e.g., crosslinkable-graftable moieties such asvinyl and allyl), halo (especially chloro and/or bromo), nitro, alkoxyand the like. Unsubstituted phenylene radicals are preferred. Both A¹and A² are preferably p-phenylene, although both may be o- orm-phenylene or one o- or m-phenylene and the other p-phenylene.

The bridging radical, Y², is one in which one or two atoms, preferablyone, separate A¹ from A². It is most often a hydrocarbon radical andparticularly a saturated radical such as methylene, cyclohexylmethylene,2-[2.2.1]-bicycloheptylmethylene, ethylene, isopropylidene,neopentylidene, cyclohexylidene, cyclopentadecylidene, cyclododecylideneor adamantylidene, especially a gem-alkylene radical. Also included,however, are unsaturated radicals and radicals which are entirely orpartially composed of atoms other than carbon and hydrogen. Examples ofsuch radicals are 2,2-dichloroethylidene, carbonyl, thio and sulfone.For reasons of availability and particular suitability for the purposesof this invention, the preferred radical of formula II is the2,2-bis(4-phenylene)propane radical, which is derived from bisphenol Aand in which Y² is isopropylidene and A¹ and A² are each p-phenylene.

As noted, each Y¹ value is independently oxygen or sulfur. Most often,all Y¹ values are oxygen and the corresponding compositions are cyclicpolycarbonate oligomers.

The cyclic oligomers have degrees of polymerization from 2 to about 30.Cyclic oligomer mixtures are preferred, especially those in which thevarious molecular species have degrees of polymerization up to about 20,with a major proportion being up to about 12 and a still largerproportion up to about 15. Such mixtures have relatively low meltingpoints as compared to single compounds such as the corresponding cyclictrimer. The cyclic oligomer mixtures are generally liquid attemperatures above 300° C., most often at temperatures above 225° C. andfrequently above 200° C.

The cyclic oligomer mixtures should generally contain low proportions oflinear oligomers. In general, no more than about 10% by weight, and mostoften no more than about 5%, of such linear oligomers should be present.The mixtures also usually contain low percentages (frequently less than30% and preferably no higher than about 20%) of polymers (linear orcyclic) having a degree of polymerization greater than about 30. Theseproperties, coupled with the relatively low melting points andviscosities of the cyclic oligomer mixtures, contribute to their utilityas resin precursors, especially for high molecular weight resins.

Suitable cyclic oligomer mixtures may be prepared by a condensationreaction involving at least one compound selected from the groupconsisting of bishaloformates and thiol analogs thereof, said compoundshaving formula III, or a mixture thereof with at least one bis(activehydrogen) compound having formula IV, wherein R¹ and Y¹ are as definedhereinabove, X is chlorine or bromine, and each Y³ is independentlysulfur when the corresponding R¹ is aliphatic or alicyclic and oxygen orsulfur when the corresponding R¹ is aromatic. (The compound of formulaIII or mixture thereof with that of formula IV is frequently referred tohereinafter as "bishaloformate composition" or "bischloroformatecomposition".) The condensation reaction typically takes placeinterfacially when a solution of said compound in a substantiallynon-polar organic liquid is contacted with a tertiary amine from aspecific class and an aqueous alkali metal hydroxide solution.

In addition to compounds of formula III and, optionally, formula IV, thebishaloformate composition may also contain other compounds, includingoligomers of formula V, wherein R¹, Y¹ and X are as previously definedand n is a small number, typically about 1-4.

While the X values in formula III may be chlorine or bromine, thebischloroformates, in which X is chlorine, are most readily availableand their use is therefore preferred. (Frequent reference tobischloroformates will be made hereinafter, but it should be understoodthat other bishaloformates may be substituted therefor as appropriate.)Suitable bis(active hydrogen) compounds of formula IV include diols andthiol analogs thereof having divalent radicals of formula II which aredifferent from the corresponding divalent radicals in the compound offormula III, as well as other dihydroxyaromatic compounds and thiolanalogs thereof. When such bis(active hydrogen) compounds are present,they generally comprise up to about 50%, most often up to about 20% andpreferably up to about 10%, of the bischloroformate mixture. Mostpreferably, however, said mixture consists essentially ofbischloroformates. Any cyclic oligomers containing divalent aliphaticradicals (or their vinylogs) flanked by two oxygen atoms are prepared byusing a mixture of compounds of formulas III and IV.

The tertiary amines useful in the oligomer formation reaction("tertiary" in this context denoting the absence of N--H bonds)generally comprise those which are oleophilic; i.e., which are solublein and highly active in organic media, especially those used in theoligomer preparation method, and in particular those which are usefulfor the formation of polycarbonates. Reference is made, for example, tothe tertiary amines disclosed in the aforementioned U.S. Pat. No.4,217,438 and in U.S. Pat. No. 4,368,315, the disclosure of which isalso incorporated by reference herein. They include aliphatic aminessuch as triethylamine, tri-n-propylamine, diethyl-n-propylamine andtri-n-butylamine and highly nucleophilic heterocyclic amines such as4-dimethylaminopyridine (which, for the purposes of this invention,contains only one active amine group). The preferred amines are thosewhich dissolve preferentially in the organic phase of the reactionsystem; that is, for which the organic-aqueous partition coefficient isgreater than 1. This is true because intimate contact between the amineand bischloroformate composition is essential for the formation of thecyclic oligomer mixture. For the most part, such amines contain at leastabout 6 and preferably about 6-14 carbon atoms.

The most useful amines are trialkylamines containing no branching on thecarbon atoms in the 1- and 2- positions. Especially preferred aretri-n-alkylamines in which the alkyl groups contain up to about 4 carbonatoms. Triethylamine is most preferred by reason of its particularavailability, low cost, and effectiveness in the preparation of productscontaining low percentages of linear oligomers and high polymers.

Also employed in the oligomer formation reaction is an aqueous alkalimetal hydroxide solution. It is most often lithium, sodium or potassiumhydroxide, with sodium hydroxide being preferred because of itsavailability and relatively low cost. The concentration of said solutionis about 0.2-10M and preferably no higher than about 3M.

The fourth essential component in the cyclic oligomer preparation methodis a substantially non-polar organic liquid which forms a two-phasesystem with water. The identity of the liquid is not critical, providedit possesses the stated properties. Illustrative liquids are aromatichydrocarbons such as toluene and xylene; substituted aromatichydrocarbons such as chlorobenzene, o-dichlorobenzene and nitrobenzene;chlorinated aliphatic hydrocarbons such as chloroform and methylenechloride; and mixtures of the foregoing with ethers such astetrahydrofuran.

To prepare the cyclic oligomer mixture according to the above-describedmethod, in the first step the reagents and components are placed incontact under conditions wherein the bischloroformate composition ispresent in high dilution, or equivalent conditions. Actual high dilutionconditions, requiring a large proportion of organic liquid, may beemployed but are usually not preferred for cost and convenience reasons.Instead, simulated high dilution conditions known to those skilled inthe art may be employed. For example, in one embodiment of the methodthe bischloroformate composition or a mixture thereof with the amine isadded gradually to a mixture of the other materials. It is within thescope of this embodiment to incorporate the amine in the mixture towhich the bischloroformate is added, or to add it gradually, either inadmixture therewith or separately. Continuous or incremental addition ofamine is frequently preferred, whereupon the cyclic oligomer mixture isobtained in relatively pure form and in high yield.

Although addition of bischloroformate composition neat (i.e., withoutsolvents) is within the scope of this embodiment, it is frequentlyinconvenient because many bischloroformates are solids. Therefore, it ispreferably added as a solution in a portion of the organic liquid,especially when it consists essentially of bischloroformate. Theproportion of organic liquid used for this purpose is not critical;about 25-75% by weight, and especially about 40-60%, is preferred.

The reaction temperature is generally in the range of about 0°-50° C. Itis most often about 0°-40° C. and preferably 20°-40° C.

For maximization of the yield and purity of cyclic oligomers as opposedto polymer and insoluble and/or intractable by-products, it is preferredto use not more than about 0.7 mole of bischloroformate composition(calculated as bisphenol bischloroformate) per liter of organic liquidpresent in the reaction system, including any liquid used to dissolvesaid composition. Preferably, about 0.003-0.6 mole thereof is used whenit consists entirely of bischloroformate, and no more than about 0.5mole is used when it is a mixture of compounds of formulas III and IV.It should be noted that this is not a molar concentration in the organicliquid when the bischloroformate composition is added gradually, sincesaid composition is consumed as it is added to the reaction system.

The molar proportions of the reagents constitute another importantfeature for yield and purity maximization. The preferred molar ratio ofamine to bischloroformate composition (calculated as bisphenolbischloroformate) is about 0.1-1.0:1 and most often about 0.2-0.6:1. Thepreferred molar ratio of alkali metal hydroxide to said composition isabout 1.5-3:1 and most often about 2-3:1.

The second step of the cyclic oligomer preparation method is theseparation of the oligomer mixture from at least a portion of thepolymer and insoluble material present. When other reagents are added tothe alkali metal hydroxide solution and the preferred conditions andmaterial proportions are otherwise employed, the cyclic oligomer mixture(obtained as a solution in the organic liquid) typically contains lessthan 30% by weight and frequently less than about 20% of polymer andinsoluble material. When all of the preferred conditions are employed,the product may contain 10% or even less of such material. Depending onthe intended use of the cyclic oligomer mixture, the separation step maythen be unnecessary.

Therefore, a highly preferred method for preparing the cyclic oligomermixture comprises the single step of conducting the reaction using atleast one aliphatic or heterocyclic tertiary amine which, under thereaction conditions, dissolves preferentially in the organic phase ofthe reaction system, and gradually adding all the reagentssimultaneously to a substantially non-polar organic liquid or a mixtureof said liquid with water, said liquid or mixture being maintained at atemperature in the range of about 0°-50° C.; the amount ofbischloroformate composition used being up to about 0.7 mole for eachliter of said organic liquid present in the reaction system, and themolar proportions of amine and alkali metal hydroxide tobischloroformate composition being approximately 0.2-1.0:1 and 2-3:1,respectively; and recovering the cyclic oligomers thus formed.

As in the embodiment previously described, another portion of saidliquid may serve as a solvent for the bischloroformate composition.Addition of each reagent is preferably continuous, but may beincremental for any or all of said reagents.

Among the principal advantages of this preferred embodiment are thenon-criticality of the degree of dilution of the reagents and theability to complete the addition and reaction in a relatively shorttime, regardless of reaction scale. It ordinarily takes only about 25-30minutes to complete cyclic oligomer preparation by this method, and thecyclic oligomer yield may be 85-90% or more. The crude product usuallyalso contains only minor amounts of high molecular weight linearpolycarbonates as by-products. By contrast, use of a less preferredembodiment may, depending on reaction scale, require an addition periodas long as 8-10 hours and the crude product may contain substantialproportions of linear by-products with molecular weights of about4,000-10,000, which, if not removed, may interfere with subsequentpolymerization of the cyclic oligomers by acting as chain transferagents.

It is believed that the advantageous results obtained by employing thepreferred embodiment are a result of the relatively low pH of thereaction mixture, typically about 9-10. When bischloroformatecomposition (and optionally amine) is added to alkali metal hydroxide,on the other hand, the initial pH is on the order of 14.

When the polymer separation step is necessary, the unwanted impuritiesmay be removed in the necessary amounts by conventional operations suchas combining the solution with a non-solvent for said impurities.Illustrative non-solvents include ketones such as acetone and methylisobutyl ketone and esters such as methyl acetate and ethyl acetate.Acetone is a particularly preferred non-solvent.

Recovery of the cyclic oligomers normally means merely separating thesame from diluent (by known methods such as vacuum evaporation) and,optionally, from high polymer and other impurities. As previouslysuggested, the degree of sophistication of recovery will depend on suchvariables as the intended end use of the product.

The preparation of cyclic oligomer mixtures useful in this invention isillustrated by the following examples. All parts and percentages in theexamples herein are by weight unless otherwise indicated. Temperaturesare in degrees Celsius. Molecular weights, whenever referred to herein,are weight average unless otherwise indicated and were determined by gelpermeation chromatography relative to polystyrene.

EXAMPLES 1-18

Bisphenol A bischloroformate was reacted with aqueous sodium hydroxideand triethylamine in an organic liquid (chloroform in Example 7,methylene chloride in all other examples) according to the followingprocedure: The bischloroformate was dissolved in half the amount oforganic liquid employed and was added gradually, with slow stirring, tothe balance of the reaction mixture. In Examples 1-10 and 12, thetriethylamine was all originally present in the reaction vessel; inExamples 14-16, it was added gradually at the same time as thebischloroformates; and in Examples 11, 13, 17 and 18, it was addedincrementally at the beginning of bischloroformate addition and atintervals of 20% during said addition. The amount of sodium hydroxideused was 2.4 moles per mole of bischloroformate. After all thebischloroformate had been added, the mixture was stirred for about 2minutes and the reaction was quenched by the addition of a slight excessof 1M aqueous hydrochloric acid. The solution in the organic liquid waswashed twice with dilute aqueous hydrochloric acid, dried by filtrationthrough phase separation paper and evaporated under vacuum. The residuewas dissolved in tetrahydrofuran and high polymers were precipitated byaddition of acetone.

The reaction conditions for Examples 1-18 are listed in Table 1 togetherwith the approximate percentage (by weight) of cyclic polycarbonateoligomer present in the product before high polymer precipitation. Theweight average molecular weights of the cyclic oligomer mixtures wereapproximately 1300, corresponding to an average degree of polymerizationof about 5.1.

                                      TABLE I                                     __________________________________________________________________________                                Molar ratio,                                           Bischloroformate                                                                       Bischloroformate                                                                            amine:                                                 amt., mmole/l.                                                                         amt.,    NaOH bis-chloro-   Addition                                                                            % oligomer                    Example                                                                            org. liquid                                                                            total mmol.                                                                            molarity                                                                           formate                                                                              Temperature                                                                          time, min.                                                                          in product                    __________________________________________________________________________     1   100      2         0.313                                                                             0.5    20     30    97                             2   100      2         0.625                                                                             0.5    20     30    95                             3   100      2        2.5  0.5    35     55    93                             4   100      2        2.5  0.5     0     30    77                             5   100      2        2.5  0.5    20     30    87                             6   100      2        2.5  0.5    35     30    78                             7   100      2        2.5  0.5    50     30    88                             8   100      2        2.5   0.25  20     30    74                             9   100      1        2.5  0.2    20     15    75                            10   200      4        2.5  0.5    20     30    88                            11   500      10       2.5   0.25  25     105   83                            12   500      10       2.5   0.25  25     105   78                            13   500      10       2.5   0.25  25     105   83                            14   500      10       2.5   0.25  25     105   87                            15   500      10       2.5   0.29  30     90    78                            16   500      10       2.5   0.25  30     20    75                            17   500      10       2.5   0.25  40-45  105   79                            18   500      10       2.5  0.4    25     105   79                            __________________________________________________________________________

EXAMPLE 19

Bisphenol A bischloroformate (2.0 mmol.) was reacted with aqueous sodiumhydroxide and 4-dimethylaminopyridine in methylene chloride. Theprocedure employed was that of Example 1, except that 66.67 mmol. ofbisphenol A per liter of methylene chloride was employed, the aqueoussodium hydroxide concentration was 5.0M and the reaction temperature wasabout 25° C. The product comprised 85% cyclic oligomer.

EXAMPLE 20

A solution of 1.4 mmol. of bisphenol A bischloroformate and 0.6 mmol. of1,4-benzenedimethanol bischloroformate in 10 ml. of atetrahydrofuran-methylene chloride solution comprising 10% by volumetetrahydrofuran was added over 30 minutes at 30° C, with stirring, to amixture of 10 ml. of methylene chloride, 2 ml. of 2.5M aqueous sodiumhydroxide and 1 mmol. of triethylamine. After addition was complete, themixture was washed three times with dilute aqueous hydrochloric acid andthe organic layer was separated, dried by filtration through phaseseparation paper and evaporated under vacuum. The product was thedesired mixed cyclic polycarbonate oligomer of bisphenol A andbenzene-1,4-dimethanol.

EXAMPLES 21-32

Following the procedure of Example 20, products containing at leastabout 80% mixed cyclic polycarbonate oligomers were prepared frommixtures of bisphenol A bischloroformate and the dihydroxy compounds ordithiols listed in Table II. In each case, a total of 2 mmol. of reagentA was used. The proportion of the listed dihydroxy compound or dithiolwas 10 mole percent unless otherwise indicated.

                  TABLE II                                                        ______________________________________                                        Example  Dihydroxy compound or dithiol                                        ______________________________________                                        21       1,1-Bis(4-hydroxyphenyl)cyclohexane                                  22       1,1-Bis(4-hydroxyphenyl)cyclododecane                                23       2,2-Bis(3-5-dimethyl-4-hydroxyphenyl)propane                         24       2,2-Bis(3,5-dibromo-4-hydroxyphenyl)propane                          25       Bis(4-hydroxyphenyl) sulfone                                         26       4,4'-Thiodiphenol                                                    27       Bis(3,5-dimethyl-4-hydroxyphenyl) sulfone                            28       2,2-Bis(4-hydroxyphenyl)-1,1-dichloroethylene                        29       Hydroquinone                                                         30       Hydroquinone (15 mole percent)                                       31       4,4'-Biphenyldithiol                                                 32       1,12-Dodecanedithiol                                                 ______________________________________                                    

The polymerization of the above-described cyclic polycarbonate oligomersinvolves the use of a polycarbonate formation catalyst. Such catalystsinclude various bases and Lewis acids. It is known that basic catalystsmay be used to prepare polycarbonates by the interfacial method, as wellas by transesterification and from cyclic oligomers. Such catalysts mayalso be used to polymerize the cyclic oligomer mixtures. Examplesthereof are lithium 2,2,2-trifluoroethoxide, n-butyllithium andtetramethylammonium hydroxide. Also useful are various weakly basicsalts such as sodium benzoate and lithium stearate.

Lewis acids useful as polycarbonate formation catalysts includedioctyltin oxide, triethanolaminetitanium isopropoxide,tetra(2-ethylhexyl) titanate and polyvalent metal (especially titaniumand aluminum) chelates such as bisisopropoxytitanium bisacetylacetonate(commercially available under the tradename "Tyzor AA") and thebisisopropoxyaluminum salt of ethyl acetoacetate. Among the preferredcatalysts are lithium stearate and bisisopropoxytitaniumbisacetylacetonate.

Also useful as polycarbonate formation catalysts are coordinationcompounds represented by formula VI, wherein M is one equivalent of acation other than hydrogen and Z is an aromatic radical or two Z valuestaken together form a divalent aromatic radical.

The M value may be any metal cation, with alkali metals, especiallylithium, sodium and potassium, being preferred. More desirably however,it has formula VII, wherein each R² is independently a C₁₋₄ primaryalkyl or C₆₋₁₀ aryl radical, preferably alkyl and most desirably methyl,and Q is nitrogen, phosphorus or arsenic.

The Z values in formula VI may be phenyl radicals or substituted phenylradicals wherein the substituents may be C₁₋₄ alkyl, aryl, halo, nitro,C₁₋₄ alkoxy or the like. Any substituents are preferablyelectron-withdrawing groups such as halo or nitro, but unsubstitutedphenyl radicals are most preferred. It is also possible for two Z valuestogether to form a divalent radical such as 2,2'-biphenylene.

Thus, it will be apparent to those skilled in the art that suitablecatalytic species include such compounds as lithium tetraphenylborate,sodium tetraphenylborate, sodium bis(2,2'-biphenylene)borate, potassiumtetraphenylborate, tetramethylammonium tetraphenylborate,tetra-n-butylammonium tetraphenylborate, tetramethylphosphoniumtetraphenylborate, tetra-n-butylphosphonium tetraphenylborate andtetraphenylphosphonium tetraphenylborate. As between these and similarcatalysts, the choice may be dictated by such factors as the desiredrate of reaction and the chemical nature of the oligomer compositionbeing polymerized. For the preparation of aromatic polycarbonates suchas bisphenol A polycarbonate, preferred catalysts are thetetra-n-alkylammonium and tetra-n-alkylphosphonium tetraphenylborates.Tetramethylammonium tetraphenylborate is particularly preferred becauseof its high activity, relatively low cost and ease of preparation fromtetramethylammonium hydroxide and an alkali metal tetraphenylborate.

According to the present invention, the polycarbonate formation catalystis contacted with at least one diaryl carbonate at a temperature up toabout 350° C., prior to its use to polymerize the cyclic oligomers. Thediaryl carbonates which may be used include diphenyl carbonate andsubstituted derivatives thereof, wherein the substituents may be, forexample, halo (e.g., fluoro and chloro), trifluoroalkyl, alkoxy(especially C₁₋₄ alkoxy), nitro and combinations thereof. The nature ofthe substituents on any substituted diaryl carbonate has an effect onthe polymerization rate. Thus, electron-donating substituents such asmethoxy increase the polymerization rate, while electron-withdrawingsubstituents such as nitro and halo decrease the rate thereof. For themost part, a decrease in polymerization rate is desired; therefore, thepreferred diaryl carbonates are diphenyl carbonate and the halo-,trifluoroalkyl- and nitro-substituted derivatives. Diphenyl carbonate isparticularly preferred.

The molar ratio of diaryl carbonate to catalyst is not critical but isgenerally at least about 1:1 and preferably at least about 2:1. There isno effective upper limit of said molar ratio for the purpose of catalystmodification. Since the diaryl carbonate may also serve as a chaintransfer or endcapping agent, thus controlling the molecular weight ofthe linear polycarbonate, the maximum amount thereof may be adjusted toserve this purpose. For the most part, up to about 2.5 mole percent ofdiaryl carbonate may be used, based on structural units in the oligomer.

The temperature at which catalyst modification is effected is up toabout 350° C. and is generally in the range of about 150°-300° C. Ifdesired, there may be employed a substantially inert diluent such aschlorobenzene, o-dichlorobenzene or dichlorotoluene. However, it is alsowithin the scope of the invention to modify the catalyst in the absenceof diluent; this is frequently preferred, particularly when oligomerpolymerization is effected in a mold.

Following modification of the catalyst, the polymerization of the cycliccarbonate is conducted, typically at temperatures up to about 350° C.,preferably in the range of about 200°-300° C. The preferred catalystproportion is about 0.001-0.5 mole percent, based on structural units inthe oligomer. This polymerization method is also an aspect of theinvention.

Since a "living" polymerization is involved, the molecular weight of thepolymer will vary inversely with the proportion of catalyst used. On theother hand, the reaction rate varies directly with the proportion ofcatalyst. Therefore, as said proportion is increased, the time requiredfor complete polymerization of cyclics and the molecular weight of theproduct both decrease.

Although a solvent may be used during polymerization, it is notnecessary and is frequently not preferred. It is within the scope of theinvention to conduct the polymerization in a mold to produce a moldedarticle.

For the most part, polycarbonate formation catalysts initially contactedwith diaryl carbonate remain effective, but the polymerization proceedsat a substantially slower rate. This was found to be the case with allof the catalysts listed hereinabove except the titanium-containingcatalysts, illustrated by triethanolaminetitanium isopropoxide andbisisopropoxytitanium acetylacetonate. For those catalysts, a distinctinduction period was noted. This is illustrated in a particularlydramatic manner in Example 33.

The method of this invention is illustrated by the following examples.

EXAMPLE 33

A mixture of 0.6 mg. (0.0028 mmol.) of diphenyl carbonate, 10microliters of a 0.1M solution in toluene of triethanolaminetitaniumisopropoxide (0.001 mmol.) and 2 ml. of dry 2,4-dichlorotoluene washeated under reflux for one hour, after which 1 gram (3.94 mmol.) of acyclic bisphenol A polycarbonate oligomer mixture similar to those ofExamples 1-18 was added, along with an additional 8 ml. of2,4-dichlorotoluene. Samples were removed from the mixture periodicallyand the molecular weights thereof determined by gel permeationchromatography. Comparison was made with a control in which the cycliccarbonate oligomer mixture and diphenyl carbonate were initiallycombined with 10 ml. of dichlorotoluene and allowed to reflux for 1hour, after which the catalyst solution was added.

The results are shown graphically in FIGURE IX. As is apparent, asignificant induction period was introduced by modification of thecatalyst according to the invention.

EXAMPLES 34-36

Three mixtures of 4.2 mg. (0.02 mmol.) of diphenyl carbonate and 5microliters of a 0.1M solution in toluene of triethanolaminetitaniumisopropoxide (0.0005 mmol) were heated at 200° C. for one hour. Therewas then added to each mixture 500 mg. (1.97 mmol.) of a cyclicbisphenol A polycarbonate oligomer mixture similar to those of Examples1-18. The mixtures were heated at 250° C. for various periods, afterwhich they were cooled and dissolved in methylene chloride and theproducts were precipitated into methanol. Comparison was made with acontrol in which all three reagents were initially heated together.

The following results were obtained.

    ______________________________________                                                 Heating                                                              Example  time, min.    Results                                                ______________________________________                                        Control   5            High MW polymer                                        34        5            No polymer                                             35       30            Slight MW build                                        36       60            Polymer Mw = 31,000                                    ______________________________________                                    

The induction period introduced according to this invention is apparent.

EXAMPLE 37

A mixture of 4.2 mg. (0.02 mmol.) of diphenyl carbonate, 1.9 mg. (0.005mmol.) of tetramethylammonium tetraphenylborate and 2 ml. of dry2,4-dichlorotoluene was heated under reflux for 1 hour, after which 500mg. (1.97 mmol.) of a cyclic bisphenol A polycarbonate oligomer mixturesimilar to those of Examples 1-18 was added, along with an additional 2ml. of 2,4-dichlorotoluene. Heating under reflux was continued for 1/2hour, after which the mixture was analyzed by gel permeationchromatography which showed that 75% conversion to polymer had occurred.

Comparison was made with a control in which the diphenyl carbonate,cyclic oligomer mixture and dichlorotoluene (4 ml.) were first heatedfor one hour and then tetramethylammonium tetraphenylborate was addedand heating was continued for 1/2 hour. The control showed 94%conversion to polymer.

EXAMPLE 38

The amounts of reagents employed were the same as in Example 37. Thetetramethylammonium tetraphenylborate and diphenyl carbonate were heatedin the melt (in the absence of solvent) at 200° C. for one hour, afterwhich the cyclic bisphenol A polycarbonate oligomer mixture was addedand the mixture was heated at 250° C. for five minutes. Only 8%conversion to polymer took place. Comparison was made with a control inwhich the oligomer mixture and diphenyl carbonate were first heated at200° C. for one hour and then the tetramethylammonium tetraphenylboratewas added and heating was continued at 250° C. for five minutes; 30%conversion to polymer occurred.

EXAMPLE 39

The procedure of Example 38 was repeated, substituting 500 mg. (0.04mmol.) of lithium trifluoroethoxide for the tetramethylammoniumtetraphenylborate. No polymerization was noted during the reactionperiod, while 16% polymerization was noted in the control.

What is claimed is:
 1. A method for preparing a linear polycarbonateresin which comprises:modifying a polycarbonate formation catalyst bycontacting it with at least one diaryl carbonate at a temperature in therange of about 150°-300° C., and subsequently contacting a cyclicpolycarbonate oligomer composition in which the oligomers comprisestructural units having the formula ##STR1## wherein each R¹ isindependently a divalent aliphatic, alicyclic or aromatic radical andeach Y¹ is independently oxygen or sulfur, with said modified catalystat a temperature up to about 350° C.
 2. A method according to claim 1wherein the oligomer composition is a mixture of cyclic oligomers havingdegrees of polymerization from 2 to about 20, each Y¹ is oxygen and thepolycarbonate formation catalyst is at least one base or Lewis acid. 3.A method according to claim 2 wherein each R has the formula

    --A.sup.1 --Y.sup.2 --A.sup.2 --                           (II)

wherein each of A¹ and A² is a single-ring divalent aromatic radical, Y²is a bridging radical in which one or two atoms separate A¹ from A², andthe polymerization temperature is in the range of about 200°-300° C. 4.A method according to claim 3 wherein each of A¹ and A² is p-phenylene,Y² is isopropylidene and the diaryl carbonate is diphenyl carbonate or ahalo- trifluoroalkyl-, alkoxy- or nitro-substituted derivative thereof.5. A method according to claim 4 wherein the polycarbonate formationcatalyst is at least one compound selected from the group consisting oflithium 2,2,2-trifluoroethoxide, n-butyllithium, tetramethylammoniumhydroxide, sodium benzoate, lithium stearate, dioctyltin oxide,triethanolaminetitanium isopropoxide, tetra(2-ethylhexyl) titanate,bisisopropoxytitanium bisacetylacetonate, the bisisopropoxyaluminum saltof ethyl acetoacetate, and compounds having the formula ]VI[ ##STR2##wherein M is one equivalent of a cation other than hydrogen and Z is anaromatic radical or two Z values taken together form a divalent aromaticradical.
 6. A method according to claim 5 wherein the diaryl carbonateis diphenyl carbonate which is present in an amount from about 2 molesper mole of polycarbonate formation catalyst to about 2.5 mole percentbased on oligomer structural units, and wherein a major proportion ofthe oligomers have degrees of polymerization up to about
 12. 7. A methodaccording to claim 6 wherein the polycarbonate formation catalyst is atitanium-containing catalyst.
 8. A method according to claim 7 whereinthe polycarbonate formation catalyst is triethanolaminetitaniumisopropoxide.
 9. A method according to claim 7 wherein the polycarbonateformation catalyst is bisisopropoxytitanium acetylacetonate.
 10. Amethod according to claim 6 wherein the polycarbonate formation catalysthas formula VI, Z is phenyl and M is lithium, sodium or potassium or hasthe formula

    (R.sup.2).sub.4 Q.sup.⊕  ,                             (VII)

wherein each R² is a C₁₋₄ primary alkyl or C₆₋₁₀ aryl radical and Q isnitrogen, phosphorus or arsenic.